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| Q&A
for 14CL |
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Hi Dr. Pang I was wondering for the vitamin C lab, what was the purpose of adding KI, NaHCO3, and H2SO4?
H2SO4 acts as a catalyst. Bicarbonate is used to provided an inert environment for the reaction. KI is used together with KIO3 to generate the I3(-).
professor pang, i was wondering when calculating the percent by mass of ascorbic acid in the vitamin C sample is the ascorbic acid the 0.1 grams we used in each trial. thanks
You should have the precise value of the mass recorded for each trial in your lab notebook.
It doesn't make sense to me why we would use the Na2S2O3 volume added from the point where we add the startch (blue) to when it turns colorless; what about the Na2S2O3 that was added before that, why do we ignore that volume?
The end point volume is defined as the volume when the solution turned to colorless minus the INITIAL volume. You do include all the volume of thiosulfate you added during the titration.
Prof ~ Do you use the moles of KIO3 you added to the solution, which is 20ml multiplied by the molar conc. to determine the moles of na2s2o3 present?
The molarity of KIO3 togther with the amount of KIO3 used during the standardization will give you the moles of KIO3.
Prof ~ Do determine the Molar Conc. of Na2S2O3 do we just find the moles of KIO3 using the molar conc. from part 1 and multiply by .02L and use this number of moles times 6 (from the mole ratio) and put it over the total volume to get to the endpoint?
Refer to eq. 2 listed on the experimental handout. At the end point, moles of KIO3 equal to the moles of thiosulfate.
When calculating the endpoint volumes for the vitamin C lab, do we use the final volume after the solution turns clear minus the initial volume before titration, or do we just see how many ml were added to turn the solution from blue to clear?
The end point volume is the point when the indicator changed from blue to colorless.
Will we be given the DEPT charts for the exam? Thanks =)
Yes, I will.
Professor Pang, I am confused about which titrant volume you mean. The note says that the end point volume is from the point where it turns blue (after some Na2S2O3 has been added) to when its colorless, but I don't see why we would use that volume. Later you state that we use the initial and end point volumes, which I believe is the reading before any titrant is added and the end point being the total volume added to change it to colorless, and this makes more sense. Which one is it? Also, can we use MaVa=MbVb to find the concentration of Na2S2O3, taking into account the 1:6 ratio of KIO3 moles to Na2S2O3 moles?
The end point volume is the point when the indicator (i.e. the starch) changed its color from blue to colorless. This indicates that there is no more I3(-) in the solution. You can equate the moles to solve for the unknown quantity.
Hi Professor Pang, If we want to dispute our CPR score, how should we go about doing this? Should we come to your office hours? Thanks!
You can simply email me directly with your reasoning.
Hi Dr. Pang, I am having trouble identifying the number of hydrogens on each carbon in spectrum 1 (question 3 of lab 15). Can you provide a hint as to how we are to go about doing so? Also, will we be needing a coat and googles for the last two lab meetings? Thanks.
Refer to your lecture guide on the for the 13C NMR tables and the DEPT 45, 90 & 135 chemical shift tables.
Professor, How do we calculate excess moles I3? Do we multiply [KIO3] by mL of Na2S2O3 from Period 1 or 2? And if so, is it the average of the three titrations? Thanks a lot.
Refer to equation #4.
For prelab of NMR spec. #1, where is 2,3-dimethylbutene? Is it between carbon 2 and 3? or 1 and 2?
The name already indicated to you the locations of the two methyl groups in the compound.
For the Vitamin C lab, I am confused as to what equations to use to determine the concentrations of the solutions. Do we use eq 2 only to determine the concentration of Na2S2O3? But then what about the solid KI we added? Won't that affect the real amount of IO3- that reacts with S2O32-? For the ascorbic acid, which equations are relevant? I guess I am basically confused about the order of the reactions and which apply to determining the concentration of Na2S2O3 and which apply to the concentration of vit C.
All the main reactions you need for the experiment are listed from eq.1 to eq. 4.
For the Vit C lab error analysis sect, is the % RAD for the sodium thiosulfate soln done using the Molar Concentrations or using the number of moles for sodium thiosulfate soln....Also for the disc sect do we just do our own research online or will there be further direc???
The %RAD is for the concentration of the thiosulfate solution. For the discussion section, you simply do your own "research" either online or from any resources you can find from the library.
Hi prof pang. For the Titrimetric Analysis of VItamin C... how do we reference? there are no authors, page number, or title for where it came from. Is it the same as the labbook? Chemistry Experiments for LS majors 2nd edition by russell?
You should cite the laboratory manual as well as the experimental handout.
Professor Pang, will the final be a written exam or on the computer? Thanks =)
14CL final exam will be a written exam (similar to the style of the midterm).
Professor Pang, sorry another quick question about cpr. How do I know if have mastered my retake. The screen immediately proceeds to say "You cannot view your calibration results until Wednesday, November 30, 2005 at 11:00:00 PM" without first telling me if I have mastered my retakes.
I believe if you did not master the calibrations, the CPR program will indicate to you that you will need to retake one or more of the calibrations. You will not see the answers but the program will let you know which calibration(s) you need to retake if necessary.
professor pang, For cpr if I had to retake the calibrations do I lose points?
As you as you master all the calibrations before the due date, you don't lose points for the calibrations part of the assignment.
Professor Pang, is there an actual pre-lab due this week? It said to include a flow chart for the entire experiment last time...but i'm not sure...Thanks for your time! =)
There is no pre-lab report that is due this coming week (11/22 & 23). You do have a post-lab report that is due this coming week.
Is Camphor lab an individual lab (turning in separate lab reports) although I did the lab with my partner?
It's an INDIVIDUAL report as stated on VOH guidelines despite the fact that two students worked together during the experiment.
Professor, as a follow up to the last question, what is the signal divide?
I am not sure whether I understand your question. If you are wondering why there are two signals, you should go back and take a look at the lecture note and the concepts behind gas chromatography.
Dr. Pang, what does it mean if you have a slightly lower retention time then the standard retention times for ethyl acetate and toluene?
Depending on the differences, sometime a very small change in temperature on the GC or a change in flow rate will cause a change in retention time. Normally, the change in those cases should be very small.
In the distillation/GC postlab we are asked to compare the retention time of the standard toluene/ethyl acetate mixture with that of our samples. What is the retention time of the standard toluene/ethyl acetate mixture? Or where can I find this info?----Thanks
The standard graph is posted on the GC. I am sure your TA has reminded all of you to measure the distances between the t=0 and the ethyl acetate and toluene signal. Using the speed of the recorder and the distance, you can calculate the standard retention time for ethyl acetate and toluene.
Hi Dr. Pang. For the GC/distillation postlab, how do we calculate retention times? Is it (peak width)/(4 cm/min) converted to seconds?
It is the distance from t=0 to the center of the signal divide by the speed of the recorder (i.e. 4cm / min).
Hello Prof. I lost my lab protocols that were handed out in lab for the two experiments: oxidation and spinach. I was wondering if you can post them on VOH so I can download them and print them. Thanks.
You can drop by my office on Monday to pick up those handouts.
For the Caffeine Post lab in regards to calculating the % caffeine recovered, do we use the whole weight of the 2 tea bags that we measured or do we multiply that number by 2-4%, the amount of caffeine present in the tea bags? Thanks.
Simply divide the grams of caffeine by the weight of the tea bags to find the %caffeine extracted from the tea bags.
In review question number 4, why is it that we add either NaOH or HCl? Is the result to make one of the molecules more polar by making it charged (and able to H-bond in the case of pyridine) and hence to allow it to dissolve in a polar solvent like water?
Pyridine only has a slight solubility in water. However, if HCl is added to pyridine, it will become a salt (polar). Now the salt of the pyridine will migrate to the aqueous solution.
Hi Dr. Pang. Sorry, I still can't figure out how to find the pH at 28 ml for problem 1ii. I tried using an ICE box instead of the HH equation but I still can't get the right answer. What values do we use for our ICE box?
You should come by and see me during my office hours. It is a lot easier to go through the concepts during that time.
From Aldol Condensation reaction, out of three forms of dibenzalacetone, which one is the most preferred/ stable? And why? I thought that trans trans has the least hindrance, but from my molecular workshop, it turned out to be the trans cis has the least. Thank you.
The trans-trans isomer should be the most stable since it has the least steric effect between to two phenyl groups.
Will we be given the IR table (indicating what groups will be found in certain regions of the spectrum?)
IR correlation chart will be provided on the exam.
Hi Dr. Pang, On the practice midterm we were titrating with an acid, so we used ph=pka+log(b/a). If we were titrating with a base would we use poh=pkb+log(b/a) instead and then get the ph=14-poh? Is this correct? thanks for all your help..I greatly appreciate it!
You can still use the same formula since there is a relationship between pKa and pKb. (14= pKa + pKb)
Hi Professor Pang, for the first lab of this week T/W, we're only doing stuff in the computer lab, so we don't need goggles/lab coat/closed toe shoes, right? Thanks =)
Correct.
professor pang, for the midterm do we need to memorize the ir table?
IR correlation chart will be provided on the exam.
Hi Dr. Pang I'm a little confused about setting up the ICE box. When do I know that I need to reverse the equation? I know that I need to reverse it at the equivalence point. Or do I reverse it at every volume except when using the Henderson formula?
You shouldn't think about "reversing" a reaction as part of the understanding. Instead always think about the chemical reaction. Then you can decide what you need to do in order to find the pH.
Hi Dr. Pang. For practice problem #7 for spectrum A, since the structure is cyclohexane, are we just ignoring the peaks at 1450 and 850-950?
The signal around 1450 cm(-1) simply corresponds to the alkane CH bending mode in spectrum A.
I am a little unclear on what the heat of formation is exactly, and how from that number you would able to tell which isomer would be formed...for example, does a high heat of formation denote something that has a lot of strain so would be less likely to form, and a low heat of formation denote the opposite...or do I have that wrong?
The standard heat of reaction is a measure of the change in total bond energy during a reaction. A negative heat of reaction signifies an exothermic process. In general, it is sometimes related to Gibbs free-energy which is a measure of whether a certain reaction can occur spontaneously or not. A negative Gibbs free-energy indicates that the reaction can occur spontaneously.
The CPR source link for "Absorption and the color wheel" was not working. Who should i report it to?
The first two resources are more than enough to master the concepts for the assignment.
For the caff. extraction pre-lab, part (c), how do you know that Wi= 5g? and shouldn't W1(n)=amnt left in solvent=1-extracted amount= 1-.90 here so: =.10?
I don't think I gave the value of Wi for that question. However, you can always rearrange the question and get W1(n)/Wi for the left side of the expression. This is now a fraction which represents the relative amount of material that remain in solvent 1 after n extraction. If you want 90% of the material in solvent 2 then the ratio of W1(n)/Wi should be 10% which is 0.1.
will we be given the errors for pipet, buret, etc, or do we need to know them?
If needed, they will be provided on the exam.
I am confused on how to calculate the answers for 3b...is there an example of this somewhere (other than the book bc I have the wrong edition =( )...ie. in the lecture notes that i might have missed?
It is a simple mole ratios problem. The mole ratio of benzoic acid to bicarbonate is 1:1. Therefore, knowing the moles of benzoic acid will allow you to find the moles of bicarbonate.
I was wondering if we use the henderson hasselbach equation for 2 ii.
Correct! If you set up the ICE box correctly, you will see that the resulting mixture is a buffer.
Hi, for question 1-ii on the practice midterm, I get different results depending on whether I use the Handerson-Hasselbach or solve the equilibrium using an ICE box. My understanding is that the H-H equation is an approximation in this case (it assumes that the initial concentrations of weak acid / weak base does not change. (1) Is it right to assume that teh ICE box gives the more accurate result? (2)If so, how "far" can we use the H-H equation (1 pH above/below?) (3) Is that what explains the discrepancy in results based on teh method used? (4)If so, is it a bit of an overkill to write 3 sig-figs in the answer key ?
You can safely use Henderson=Hasselbalch equation as long as the Ka for the weak acid is less than or equal to around 10E-5. Remember that the FIRST digit in pH doesn't count as significant figure. A pH = 2.54 actually only have 2 significant figures. The "2" is the "place holder" for the log pH.
When I use a 1:1 ratio for the HCL and sodium benzoate, can I assume that the moles of benzoic acid initially in solution is the same amount of moles of sodium benzoate produced? Also, what is w/v%?
w/v% was covered in 14BL. It is express as (grams of solute / 100mL of solvent).
For question #3, can we assume that the ratio between HCL and sodium bezoate is 1:1, since the sodium benzoate needs to be protonated only once? Also, how does w/v% break down into g/liter?
CORRECT! The balanced chemical reaction is listed on the answer sheet.
Hi Dr. Pang For practice midterm 2(i), I understand that K2HPO4 is an intermediate ion. And the equation for the pH of an intermediate is (pKax+pKay)/2. Does it work for an intermediate ion at any volume because it's the only ion left in solution at the equivalence point? Also can you explain why K2HPO4 would not be in a buffer solution? Wouldn't the solution contain H2PO4 2- , which is a weak acid?
For all practical reason, as long as the concentration for the intermediate ion is not too low, the average of the pKas will give you the pH for the solution. Your second question is a very GOOD question! There are actually two equilibria for HPO4(2-) by interacting it with water. One is the formation of H2PO4(-) and the other is the formation of PO4(3-). This is why we identified HPO4(2-) as an intermediate ion rather than a buffer solution. As a matter of fact, the expression of the average of the pKas to find the pH for an intermediate ion stem from the concepts in buffer. Except in this case, we are dealing with two different sets of competing equilibria.
On the practice prob's for midterm...1(ii), I understand that you use Henderson-Hassel. for 7mL etc, however, I'm confused on how to find the [A-] and [HA]...the moles should stay the same throughout (.oo1), right? but in terms of the liters...we take into account total liters (of both HCl and Na3AsO4) for each concentration, or just the liters of that specific one?
No, the mole ratio cannot stay the same throughout the reaction. Think about it if H+ is added to a solution of AsO4(3-), how can the ratio of AsO4(3-) and HAsO4(2-) stay the same throught the reaction? One is being reacted, namely the AsO4(3-) and the one is being created (HAsO4(3-). Set up the ICE box for the reaction of H+ with AsO43- and you will see the mole ratios do not stay the same (unless you are talking about ONE SPECIFIC POINT on a titration graph) throughout the ptocess of titration. Refer to your pre-lab study questions for the reactions. You should already had the reaction since this is one of the questions that I assigned for the pre-lab report.
Since for question 2i, HPO4 is an intermediate, can you explain why we would only use the average of pka2 and pka3 to find the PH and not use pka1?
Write out the definitions for the THREE acid dissociation constants for phosphoric acid. Once you have done that, you will be able to see which two Kas to use.
Thanks. But, if I use the henderson hasselbach equation, how do i calculate the ratio of Conjugate acid to base? Thanx again
Just like in 14A and 14BL, you simply set up an ICE box for the reaction. Please review the basic materials on buffer and ICE box from your general chemistry text. Those topics were discussed extensively in 14A and 14BL.
For question #2 on the practice exam, do i use pOH=pKb-log ([acid]/[base]). If so, how do I obtain the ratio of concentration of acid/base
Think about the properties of K2HPO4. You can only use the Henderson-Hasselbalch equation if the solution is a buffer. Think about whether K2HPO4 is really a buffer solution or not.
For Question #1b on the practice midterm, for the 7ml, do we use the henderson hasselbach equation? Thanx
CORRECT! Very good! You can simply find the pH by using the Henderson-Hasselbalch equation (14A / 14BL).
Prof. Pang, Do we need to keep significant figures when we're solving problems in the midterm or the final?
Yes.
Hi Professor. I was wondering if the practice midterm solutions will be posted. This is what I heard from my TA, so I just wanted a date. If not, I had a question on 2) i, ii, and iii. Which Ka should I use for i? Common sense told me Ka3, but it didn't work even when I adjusted for Kb. Am i overlooking something? Thanks./
Please refer to the previous comments posted on VOH.
For the practice midterm, #3b, dont we need to know the partition coefficient for these two solutions to solve this? If not, could you please give me some guidance as to how to approach this problem. Thank you!
Refer to the previous comments posted on VOH.
Prof. Pang, for question #2 on the practice midterm, which ka do I use to find the initial PH of 50 ml of .5M k2hpo4? Additionally, is there any way you can write out this reaction, because I am unclear as to whether this acts like an acid or a base.
First, you need to write out ALL the definitions of the acid dissociation constants for phosphoric acid (i.e. Ka1, Ka2 and Ka3). Next, think about whether K2HPO4 is a weak acid, weak base, a buffer or an intermediate ion. We have discussed in lecture how to prove whether a certain chemical species is an intermediate ion. With the definitions of Kas, you should able to see easily which Ka or Kas to use.
For Practice Problems for Midterm #1-i, I get 8.93 for pH at 20mL of NaOH instead of 8.43 (I repeated this calculation for several times). Would you check the answer again? Thank you.
It is correct. Make sure you use the correct total volume (i.e. 30mL) for the question.
Hi Dr. Pang, will you post a solutions key to the practice problems for the midterm?
Please refer to the previous VOH comment
Hi Professor Pang, will we have to memorize the equations that we have used (i.e. HCl normalization equation, extraction equations) in the prelabs and postlabs for the midterm? Or will all the equations that we need be given to us with the exam? Thanks!
Equations for single and multiple extractions or normalization equation will be provided on the exam if they are needed.
hi professor, on the midterm review handout, for problem 1-ii, when we are finding out the pH after the addition of 28 mL of HCl, the answer guide says that a ICE box is necessary; i am confused as to how to set it up; any clarification on the initial concentrations would be appreciated; thanks
Instead of using the Henderson-Hasselbalch equation for finding out buffer pH, you will need to set up an ICE for the buffer in order to find the pH. This is becasue the Ka is higher than 10E-5. In 14A/14BL, you should have learned that one of the limitation of the Henderson-Hasselbalch equation is that it will fail if the Ka for the weak acid in the buffer is higher than 10E-5. For that question, simply set up the following ICE box, H3AsO4 + HOH = H2AsO4- + H3O+
"What factor determines the magnitude of the partition coefficient?".....it is the electronegativity/polarity/dielectric constants, right? thanks =)
In general, dielectric constant give a relative scale of polarity. It is related to the electronegativity of atoms in the molecule.
Dr. Pang can you please put up solutions to the midterm review handout. Thanks
The answer sheet is attached to the last page of the practice problems set. For question on acid base, please review the lecture guides, your pre-lab study questions and your 14A/14BL materials on equilibria.
Dr. Pang, In order to do part b of question 3 on the midterm practice problems, do you need either the partition coefficient between the ether solution and the aqueous sodium bicarbonate or the solubility of the benzoic acid in both the ether solution and the aqueous sodium bicarbonate? If not, what do you use to solve the problem? Thanks! Laura
Simply use the mole ratios between benzoic acid and the bicarbonate to solve the problem. The balanced chemical reaction was already written for you on the answer sheet. You can simply see that the mole ratio is 1:1 between benzoic acid and bicarbonate. In other words, you don't need any partition coefficient for question #3.
Referring to lecture 1 notes, (1)What's the pH of a .050M H2L+ solution?.....after using the quadratic equation, why did we use the positive x value instead of the negative? Thank you for your time =)
The product concentration INCREASES during the reaction. Besides, think about physically what it means if you have a term "-x" (instead of "+x")for a final product.
Hi Dr. Pang, I was wondering what you meant by this question on the Molecular Modeling Workshop: "What molecular fragments from the "Tool Box" would you use for the bottom or ends of the pocket to connect the two sides? Why?" Are you referring to the two sides of the pocket connecting to the phenyl rings? And does the "bottom" refer to bonding the oxygen atom? I'm attempting th make the pocket, but am having difficulty figuring out how well to make the pocket fit. Thanks!
Simply justify why your group choose certain atoms or functional groups when designing the molecular pocket. This means your group should consider the basic principles of intermolecular forces.
Professor Pang, I tried clicking on the link for the Absorption, Complementary Colors and the Color Wheel from the CPR web page but there was an error message, and I couldnt preview the site. What should I do?
Please refer to previous VOH comments.
Dr. Pang, I am not sure if it is just my computer but I was not able to get to the 4th resource link on the CPR page. I also tried it in the Life Science computer lab but was unable to get to it. Is there another link i can follow?
Refer to previous VOH comments.
hi professor, on the midterm review handout, practice problem 1, question (i); the intiial ph = 2.01 ; is this correct? my calculations show it should be 1.01
pH around 2 sounds about right. pH of 1 is too low a pH for a weak acid.
i have a question regarding the aldol condensation lab; during the purification procedure of the dibenzalacetone, what is the purpose of dissolving the product in the boiling ethanol? thank you
This is the recrystallization step. Refer to the 14BL materials and Mohrig's text for the concept in recrystallization.
Is a beam of red light considered a transparent object? Also, do transparent objects still absorb all the wavelengths of light except the wavelength of light for the color which they are?....For example, does a beam of red light absorb all the wavelengths of light except the wavelength of light for red and then transmit the wavelength of red to our eyes???...or how does it work in this case???
Refer to the CPR online resources and lecture guide for information about transmission and absoption of light.
I was just wondering when the molecular modeling lab is due? Thanks
All the report due dates are listed on the lab schedule.
Hi Dr. Pang. The link for the 3rd source for the CPR assignment does not work. Do you know the web address for the link? Thanks.
Please refer to the previous comment on VOH.
Hi Dr. Pang. I tried to start the CPR assignment but when I clicked on the last resource link it said page not found. I was able to view the first two sites and was wondering if there was anywhere else I can go to view the last one? Thanks! Elizabeth
Refer to the previous comment on VOH.
The last resource for the CPR does not open???
The first two online resources will be sufficient for the assignment.
Dr. Pang- When does CPR begin? I thought you mentioned it would start today, but I can't seem to access the assignment. Thanks!
Refer to the times and dates listed on the CPR assignment handout.
The lab schedule reads: "pre-lab due date:10/20-10/26 Caffeine Extraction (2)" 10/20-10/26 includes 2 lab periods...Does that mean that we have two pre-labs for the caffeine extraction? There is just one right(which we already turned in)? SO that would mean that the only thing we have due for the first lab this week is the amino acid post-lab, right?
I thought I stated on the pre-lab report guideline for the caffeine extraction experiment that there is only one pre-lab report for the entire experiment. For this week (10/25 - 28), there are no pre-lab reports.
Hi Dr. Pang For the amino acid postlab when we are finding the pKa1 and pKa2 from the titration, do we need to find them for all three trials then average them, or do we just pick our best full titration graph and calculate only one time?
Only use the BEST full titration graph to find the pKas.
Dr. Peng, I am working on the post-lab for the Aldol COndensation and was wondering what the molarities for the benzaldehye and acetone soluntions that we used were. THey are not listed in the lab manual and I did not record them. THanks. -Sarah Moore
From the volume and density, you can find the moles.
Do any of the TAs have office hours tomorrow?
Please check on VOH for office hours.
Dr. Pang - Is there no MSDS section for the Caffeine Extraction experiment pre-lab?
That's correct.
Hi Professor Pang, Someone told me we don't need to bring our lab coats/goggles tomorrow since we're doing molecular modeling? But I wanted to ask you, since I know we also have to do melting point determination. Can you confirm if we need our lab attire tomorrow? Thanks!
As mentioned in lecture yesterday, lab coat and goggles are NOT required for 10/18 & 19. You will start the caffeine extraction lab and take the melting point of the aldol product starting on Thursday (10/20).
Dr. Pang- For the procedure section of our pre-lab for the extraction and purification of caffine, would you like us to include the procedure for the the IR workshop in addition to the extraction and purification procedures? Thank you.
IR workshop is NOT part of the extraction of caffeine experiment. I will post the guidelines for the IR workshop later this week.
Hi Professor, i was looking at the lab schedule and it says that we have a pre-lab due for Molecular modeling due this first lab period for this week. I looked on VOH but there is no pre-lab or post lab posted up for molecular modeling, only extraction of cafeine which is the following week. Can you please clear this up for me? thanks
There is no pre-lab report for the Molecular Modeling Workshop. There is a pre-lab report due for the extraction of caffeine experiment.
IR workshop is NOT part of the extraction of caffeine experiment. I will post the guidelines for the IR workshop later this week.
Hi Professor, i was looking at the lab schedule and it says that we have a pre-lab due for Molecular modeling due this first lab period for this week. I looked on VOH but there is no pre-lab or post lab posted up for molecular modeling, only extraction of cafeine which is the following week. Can you please clear this up for me? thanks
There is no pre-lab report for the Molecular Modeling Workshop. There is a pre-lab report due for the extraction of caffeine experiment.
Hello Professor, Could you please post the Excel Titration Workshop Handout on VOH for those of us who do not still have the instructions from 14BL? If so, would you be so kind as to post them as soon as possible? Thank you for your time and consideration, Erin Peckham
If you need a copy of the EXCEL Workshop handout, cimply drop by my office.
Hi Professor! I was looking over the handout you gave us yesterday, and was wondering, for the (5) part where you have to figure out the pH after the addition of 20mL of 0.05M NaOH...where did the 1.79^10 come from...I know that is the Ka...but where did you get that # from? Thank you!
It is the pKa2 for the amino acid. The pKas were listed on the lecture notes.
I understand what to do up until 30.0mL in question 1b of the prelab. There isn't another pKa to use. There should be an equivalent point at 30.0mL and 40.0mL. How do I get the next pK value for 30.0mL and 40.0mL?
There are three different pKa's for the question. Write out the reactions and I hope you will see which pKa you suppose to use after the addition of 30mL of HCl. The concepts in this question are very similar to the one I did in lecture.
Hi Professor, I did download the Aldol Pre-Lab guidelines last week, and maybe it is just me, but the due date is not stated anywhere on it.
Due date is listed on the lab schedule. This is the same as 14BL. All the report due dates are listed on the lab schedule.
What is the due date of the Aldol Condensation pre-lab? I checked the lab schedule and pre-lab guidelines and could not find it. Is it 10/13-10/14? thanks!
The pre-lab reporyt guideline for the Aldol condensation experiment was posted on VOH since last Thursday (10/6).
Can anyone in the 8-11 or 11-2 T/R lab section switch with me? I have 6:30-9:20, but it conflicts with UCLA Drumline Practice. Please email wtom@ucla.edu, thanks. -Woodstock Tom
If anyone can make the switch, both of you will need to come and see me for proper enrollment changes. J. Pang
For question 3 of the Prelab due on Thursday, you use MaVa=MbVb to calculate the concentration of the Amino acid. There are two equivalent points, is the first or second equivalent point the Volume of the base used in the calculation of the concentration of the acid?
Refer the hint that I posted on VOH guidelines for that particular question.
Hi Dr. Pang, Just wanted to know if we are supposed to answer both section Period 1 and Period 2 for our first Prelab? Thank you.
Refer to the VOH guidelines for the assigned study questions.
What is the title of the first lab we are doing according to our notebook?
Refer to the lab schedule for the corresponding experiment in the manual.
Professor, Is there lab tomorrow morning?
I sent all the students an email about two weeks ago saying that lab attendance on 9/29 & 9/30 are mandatory. J. Pang
